The present invention generally relates to ink jet printers, and more particularly to an ink jet printer which prints information on a paper by adhering on the paper an ink which is ejected from a nozzle having an ejection hole with an extremely small diameter.
Printers can roughly be categorized into impact type printers and non-impact type printers. The impact type printer includes wire dot printers and thermal transfer printers which print images directly on the paper. On the other hand, the non-impact type printers include ink jet printer which print images without making direct contact with the paper.
In the wire dot printers and the thermal transfer printers, the paper is fed to a platen roller and the images are printed on the paper which is located on the platen roller. The transport direction of the paper is changed approximately 180.degree. to make a U-turn by the platen roller, so that the paper is positively transported to the target position.
The printers which use the platen roller can be made relatively compact because only simple parts are required. However, there is a problem in that such printers are not suited for printing images on a thick paper or an envelope, because it is difficult to positively make the thick paper and the envelope make the U-turn at the platen roller. For this reason, printers have been proposed in which the U-turn of the thick paper and the envelope is avoided by providing an independent linear paper supply path. This linear paper supply path supplies the thick paper and the envelope to a printing part of the printer in an approximately linear path without the need for the thick paper and the envelope to make the U-turn.
On the other hand, the ink jet printer prints images on the paper by ejecting droplets of an ink which is ejected from a nozzle. Since the printing part of the ink jet printer does not need to make direct contact with the paper, it is unnecessary to provide the platen. In other words, the images can be printed on the paper in a linear paper supply path.
However, in the ink jet printer, a gap between the nozzle of the printing part and the paper must always be maintained constant. For this reason, measures must be taken so that this gap is maintained constant, and a method of transporting the paper through the printing part without slack is one of such measures which are conventionally taken.
If the ink ejected from the nozzle spreads in a range in which the paper does not exist in a direction taken along the width of the paper, the ink will adhere to a guide member which is provided within this range to guide the paper. As a result, when the paper having a greater width is next transported, the ink adhered on the guide member will stain this wider paper. For this reason, it is desirable to restrict the ink ejection region.
The ink ejection region (or the printing region) can be restricted with ease in the paper transport direction by providing sensors along the transport path, for example. However, it is difficult to restrict the ink ejection region in a paper width direction which is perpendicular to the paper transport direction, because it is difficult to detect all of the various paper widths and appropriately adjust the ink ejection region for each detected paper width.
Therefore, the ink ejection region in the paper width direction is set in accordance with the maximum width of paper used by taking into consideration the printing on the paper having the maximum width. But if the printing is made using the paper having a width smaller than the maximum width, the ink is ejected to parts where no paper exist as described above, and the paper which is transported thereafter will be stained by such ink.
The ink which is adhered on the parts such as the guide member described above cannot be cleaned in a simple manner, unlike the cleaning of the platen roller using a cleaning liquid and the like.